1. Field of the Invention
The present invention relates to a method for stabilizing an aqueous solution containing a chlorine-based oxidant, and more particularly, to a method for stabilizing an aqueous solution containing a chlorine-based oxidant by pre-treating the aqueous solution with pure isocyanuric acid in the form of tablets.
2. Description of the Prior Art
Chlorine, hypochlorous acid, salts thereof, chlorinated isocyanuric acids, chlorinated hydantoins and other chlorine compounds (which are hereafter collectively referred to as chlorine-based oxidants) arc known to achieve very effective and quick control of aquatic growth (including bacteria) even if they are used in low concentrations. Because of their high oxidizing activity, these oxidants are used in waterworks as a sterilizer, in chemical plants and cooling towers as a sterilizer or algicide for circulated cooling water, in pulp mills as a slime control agent, and in swimming pools as a disinfectant.
However, the chlorine-based oxidants in aqueous solutions do not have a long keeping quality; their oxidizing ability decreases even if they are stored at ordinary temperatures, and they decompose even faster by the effect of heat, light and agitating force. Thus, if regular maintenance and constant water quality are not ensured, the intended effect of the chlorine-based oxidants is lost in a very short time and may completely fail.
Another reason for the failure of the chlorination may be that the active chlorine is bound by a variety of organic substances and thus becomes inactive. The excess chlorine remaining after bactericidal action is slowly lost, so that a proportioned addition of chlorine is required continuously.
The addition of chlorine also is disadvantageous since the taste of the water is adversely affected and can lead to the irritations of the mucous membranes. Moreover, recent studies have suggested that carcinogenic substances are produced by the addition of chlorine. However, in order to keep swimming pool water clear and free of unwanted biological growth, measured amounts of chlorine are commonly added to pool water at given intervals.
One common method of making chlorine additions is to add chlorine in the form of dry, granular calcium hypochlorite on a daily basis in a measured dose depending on the size of the pool. In order to make the task of adding chlorine easier for the pool owner, and to extend the life of the chlorine, it is common to add an isocyanurate stabilizer to the chlorinated pool water to prevent the usual rapid dissipation of chlorine, which is caused by ultraviolet light degradation to thus permit less frequent chlorine additions.
To accomplish this, chlorine has also been formulated into a slow-acting tablet form, commonly referred to as stabilized concentrated chlorinating tablets, which may conveniently allows chlorine additions on a weekly basis. A composition of tablets may consist of a mixture in compressed form of trichloroisocyanuric acid, as disclosed in U.S. Pat. No. 4,389,318, fully incorporated herein by reference. While stabilized chlorinating tablets offer a significant convenience in the maintenance of pool water, they, likewise, possess an inherent shortcoming. Commonly used isocyanurate stabilizers form isocyanuric acid in the pool water, also known as fulminuric acid.
It is observed that the isocyanuric acid concentration continues to increase in the pool water as additional isocyanurate stabilizers are added over a period of time. When the isocyanuric acid concentration in the pool water reaches a level of about 100 parts per million (PPM), a so-called “chlorine lock” occurs which renders further chlorine additions as ineffective. This condition is easily detected by way of a known pool water test wherein a conventional isocyanuric reagent turns progressively foggy within prescribed limits.
When such a chlorine lock occurs, it is common practice to drain the saturated pool water and replace it with fresh water. Of course, continued isocyanurate stabilizer additions eventually cause a similar chlorine lock to occur in the replaced water which requires yet another drain and replacement. Needless to say, this conventional practice of correcting the chlorine lock problem due to accumulated isocyanuric acid is not only inconvenient to the pool owner, requiring pool shut down, but also is expensive with respect to the high cost of replacement water which in a typical swimming pool may be on the order of about 30,000 gallons, for example.
To minimize regular addition of isocyanurate stabilizers as part of a compacted mixture including chlorine, it was suggested that isocyanuric acid be placed in the pool in granular form before the tablets are added.
One of the methods of creating a residual level of isocyanurate stabilizers in swimming pools is to simply pour granulated isocyanuric acid in the pool water. Since isocyanuric acid is not easily soluble, it tends to deposit on the bottom of the pool and to look like unwanted debris. The pool owner seeing his/her pool full of undesirable substance and not typically realizing that this is just granules to be soled in time undertakes usual steps directed to cleaning the pool and including vacuuming.
Another method of dispensing isocyanurate stabilizers is to place isocyanuric acid directly in a skimmer. Since granules are not easily dissolved, they tend to temporarily clog a pump. The pool owner alarmed by an increase of pressure indicated by a pressure gauge typically backwashes the skimmer, which leads to the result analogous to the one disclosed immediately above.
It is, therefore, desirable to provide a chlorine stabilizer that can be dissolved in pool water in a relatively short time and to provide a residual level of isocyanuric acid sufficient to prevent the reduction of the residual chlorine with an elapse of time.